Block paving manhole inspection cover

I have a square manhole cover on my blockpaved drive.Its getting too heavy for me to lift so I would like to replace this with a round pvc inspection cover and at the same time reduce the riser outlet to a diameter to match the new pvc cover. The existing riser is 470mm. Is there a way of going from a 470mm riser to a smaller diameter riser without replacing the whole of the 470mm riser.?